Turing proposed the following hypothesis:
Every 'function which would naturally be regarded as computable' can be computed by the universal Turing machine.it should be noted that there is ambiguity as to what, precisely, a function which would naturally be regarded as computable means. Due to this ambiguity, this statement is not subject to rigorous proof.
There is strong evidence for this hypothesis; many diverse models of computation have been shown to compute the same set of functions as a Turing machine, as yet there have been no counterexamples to the thesis.
This thesis gives us insight into the ``power'' of computing machines. If a computing device can solve all the problems a Turing machine can solve, then it is as powerful as a Turing machine.